oauth2: clean up the old oauth2 flow cookies without suffix

[zhaohuabing]

Hi @wbpcode, thanks for the help with the review! The backward-compatibility logic is only needed when old and new versions of Envoy coexist behind the same load balancer, which is mainly the rolling-update use case.

I plan to clean it up with a follow-up PR after the next release, so it won't confuse later contributors.

Originally posted by @zhaohuabing in #42079 (comment)

This is just a reminder for myself to clean up the old oauth2 flow cookies without suffix after the next release.

[zhaohuabing]

/assign